The end of turqouise-coloured lakes

Glaciers, even small ones, are rivers of ice. Along their way they erode and pulverize rock into tiny flour-fine particles.

Rock flour (or glacial flour) consists of fine-grained, silt-sized particles of rock, generated by mechanical grinding of bedrock by glacial erosion. Once the ice in which it is transported downward is melted, the material becomes suspended in meltwater making the water appear cloudy, which is sometimes known as glacial milk.

When the sediments enter a river, they turn the river's colour grey, light brown, iridescent blue-green, or milky white. If the river flows into a glacial lake, the lake may appear turquoise in colour as a result.

It is simply the result of reflection of light. Sunlight reflects off these tiny white particles. Because of the scattering of the light as it hits these particles, the lake takes on this turquoise colour.

But even this phenomenon might become a thing of the past as a direct result of global warming. When temperatures rise, glaciers melt and retract They may even completely disappear. No glacier means no grinding of the rock beneath it. That in turn means that no rock flour will be created that could give a mountain lake that beautiful turquoise hue.

Between the '70s and the '90s, when nobody was talking about global warming, a lot of smaller glaciers had already melted and disappeared[1]. Even now lakes are clearing up, turning into 'normal' lakes.

A clear blue lake admits much more sunlight into depths than a lake clouded with glacial flour. That's likely to bring in a different local ecology, because organisms adapted to the low light of milky waters are unlikely to survive what would be to them a harsh new glare of ultraviolet radiation. The problem is especially acute because of the speed of the transition. If you take that sunscreen away, some organisms may not be able to tolerate that increase in UV radiation. It doesn't give organisms time to adapt.

Researchers like Rolf Vinebrooke suspect that some of these lakes, at least temporarily, may be left 'biologically impoverished'.

[1] Vinebrooke: The Changing Colours of Mountain Lakes in the Twenty-First Century in State of the Mountains Report 2021. See here.